The first objective of this research project is to develop radiobiological test systems for the quantitative measurement of the toxicity of intracellularly deposited low-energy electrons and to use these systems to determine the radiotoxicity of radio-nuclides commonly used in Nuclear Medicine. The radiotoxicities of iodine-125,iodine-131 and hydrogen-3 incorporated into DNA in the form of thymidine or its analog iododeoxy-uridine have been studied and compared in cultured cell systems using clonal survival and cytogenetic aberration as end points. We shall further explore the effects of some radionuclides incorporated intracellularly but not affixed to DNA in the same test system. Our second objective is to pursue in an experimental tumor system the therapeutic implications of our observation that iodine-125 affixed to DNA is extremely radiotoxic. Antineoplastic activity has been demonstrated in an early ascities tumor model in mouse when the tumor is treated with 125IUdr, a thymidine analog labeled with I125. We shall screen for optimization of 125IUdr therapy in this experimental tumor system. BIBLIOGRAPHIC REFERENCE: W.D. Bloomer and S.J. Adelstein, Antineoplastic effect of iodine-125-labelled iododeoxyuridine. Int. J. Radiat. Biol. 27, 509-511 (1975).